The present invention relates generally to producing metal by electrolysis in a molten salt bath, and particularly to a technique for increasing the velocity of bath and metal flow while simultaneously preventing unwanted metal oxidation resulting from circulation of the molten salt into areas rich in an oxidizing agent.
For example, in cells containing molten salts of alkali metals or alkaline earth metals used in the production of aluminum by electrolysis of aluminum chloride dissolved in such salts, a vertical stack of spaced electrodes is ordinarily located within the bath of salt, such as shown in U.S. Pat. No. 3,822,195 issued in the name of Dell et al. In the electrolyzing process, chlorine gas is generated and rises to the upper portion of the cell and to an area above the upper level of the molten bath of salt, while molten metal is produced, which eventually settles to the lower portion of the cell, under force of gravity. This upward movement of chlorine gas causes circulation and an upward lift of the molten salt which, in turn, tends to carry a major amount of the produced molten metal with it. If the velocity of the upward flow is sufficiently high, the materials in the upward flow will break through the upper surface of the salt bath and enter into the area of the chlorine gas. Any metal that breaks through the upper surface of the bath and into the chlorine tends to recombine with the chlorine, the chlorine, in this case, being the oxidizing agent. The combined metal and chlorine then returns to the molten salt bath to again be decomposed in the electrolytic process. This results in reduced current efficiency of the cell since additional electrical current is required to act upon and reduce (again) the recombined metal and gas.